Hach DR/700 Procedures Manual page 187

CHLORIDE,
continued
ACCURACY CHECK
Standard Additions Method
a) Use the TenSette Pipet to add 0.1 mL, 0.2 mL and 0.3 mL of Chloride
-
Standard Solution, 1000 mg/L as Cl , to each of three 25-mL water
samples. Mix each thoroughly.
b) Analyze each sample as described above.
c) The chloride concentration should increase 4.0 mg/L for each 0.1 mL
of standard added.
d) If these increases do not occur, see Standard Additions (Section I) for
more information.
Standard Solution Method
Prepare a 10.0 mg/L chloride standard solution by diluting 5.00 mL of
Chloride Standard Solution, 1000 mg/L, to 500 mL with demineralized
water.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
-
representative lots of reagent. Testing 12.0 mg/L Cl concentration
-
samples, the standard deviation was ±0.34 mg/L Cl .
Testing zero concentration samples, the limit of detection was 0.14 mg/L
-
Cl . The limit of detection was calculated as three times the standard
deviation when testing zero concentration samples (adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
The pH of the sample after addition of reagents should be about 2. If the
sample is strongly acid or alkaline, adjust a portion of sample before
testing to a pH of about 7. Use either 5.0 N Sodium Hydroxide Standard
Solution or a 1:5 dilution of perchloric acid. Use pH paper, as most pH
electrodes will contaminate the sample with chloride.
SUMMARY OF METHOD
Chloride in the sample reacts with mercuric thiocyanate to form
mercuric chloride and liberate thiocyanate ion. Thiocyanate ions react
with the ferric ions to form an orange ferric thiocyanate complex.
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